Quenched and tempered martensite (QTM) structures were produced by tempering at 550 C in the 1200 MPa-class high strength steel with phosphorous (P) contents of 0.001 and 0.053 wt%. Charpy impact tests were performed at temperature range of 350 to -196 C on the QTM samples. The 0.001%P-QTM structure showed upper shelf energy of 97 J and a low ductile to brittle transition temperature (DBTT) of around -150 C with a transition region limited to the -100 to -196 C was observed. In the 0.053%P-QTM structure the upper shelf energy decreased from 97 J to 64 J and high DBTT of around +80 C were obtained with an extended transition region at the temperature range of 150 to -196 C. The fracture mode in the 0.001%P-QTM structure changed from ductile to cleavage and in the 0.053%P-QTM structure from ductile to intergranular and then cleavage. Thus, one can say P segregation and intergranular fracture in the 0.053%P steel increase the DBTT and extend the transition region. The critical stress for intergranular fracture was similar to or somehow lower than that for ductile fracture, and both increased with decreasing temperature. The critical stress for intergranular fracture was obtained to be 3800 at 150 C and 6000 MPa at -196 C. That is why intergranular fracture in the 0.053%P steel made the transition region wide. However, the stress for cleavage fracture was almost independent of temperature, and hence the 0.001%P steel showed a limited transition region. The critical stress for cleavage fracture was obtained to be 6000 MPa.
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